Besides the obvious quality checks, something that is quite important to me when buying second hand: in what state of assembly will the set be shipped?
Unless it's a set I'm desperately searching, I will almost certainly forego bidding if the set appears to be shipped entirely assembled.
Partially disassembled is even worse in my opinion.
Also not ...
First of all, let me congratulate you, that was indeed an amazing deal you made back then. This 8880 seems to be near mint condition, and should fetch you a nice return should you choose to sell it.
Speaking both as an experienced buyer of used LEGO sets as well as an owner of an used 8880, my personal priority list of criteria is the following: